The lock-in-amplifier is a combination of narrow band preamplification, phase sensitive detection, and DC amplification. Examples of lock-in amplifiers can be found in the "The Review of Scientific Instruments," Volume 24, number 4, (An Improved "Lock-in" Amplifier) April 1965, by Henry L. Cox, Jr., pages 307 and 308. Also Bulova American Time Products produces a Model 701 Lock-in Amplifier. It is typically used to detect very low level signals submerged in noise. The disadvantage is that in many measurements there are large and small signals. If the gain level is set to detect the small signals, the large ones overload the system. If the gain is set to detect large signals, the small ones are lost. Trial and error repetitions are required to correctly adjust.
A solution to this is to use a logarithmic amplifier. Many such slow responding amplifiers could be used after the lock-in, but then the dynamic range of the system is low. A logarithmic preamplifier can be used, but then it must be a very low noise amplifier, and the very low noise amplifier in the lock-in is wasted.